Semiconductor device

ABSTRACT

A sense amplifier capable of performing high-speed data sense operation with lower power consumption using a minuscule signal from a memory cell even in a case where a memory array voltage is reduced. A plurality of drive switches for over-driving are distributively arranged in a sense amplifier area, and a plurality of drive switches for restore operation are concentratively disposed at one end of a row of the sense amplifiers. A potential for over-driving is supplied using a meshed power line circuit. Through the use of the drive switches for over-driving, initial sense operation can be performed on data line pairs with a voltage having an amplitude larger than a data-line amplitude, allowing implementation of high-speed sense operation. The distributed arrangement of the drive switched for over-driving makes it possible to dispersively supply current in sense operation, thereby reducing a difference in sense voltage with respect to far and near positions of the sense amplifiers.

TECHNICAL FIELD

[0001] The present invention relates to a semiconductor device, and moreparticularly to a circuit for performing differential amplificationtherein.

BACKGROUND ART

[0002] In this specification, the following technical references arecited, and document numbers given thereto are hereinafter used for thesake of simplicity. [Document 1]: Japanese Unexamined Patent PublicationNo. 6 (1994) 309872 (corresponding to U.S. Pat. No. 5,412,605);[Document 2]: VLSI Memory, pp. 161-167, K. Ito, Baihuukan, 1st Issue,Nov. 5, 1994; [Document 3]: T. Yamada, et al., ISSCC91 Dig. Tech.Papers, pp. 108-109, 1991; [Document 4]: H. Hidaka, et al., IEEE Journalof Solid State Circuit, Vol 27, No. 7, (1992), pp. 1020-1027; [Document5]: Japanese Unexamined Patent Publication No. 63 (1988)- 211191;[Document 6]: S. Eto, et al., ISSCC98 Dig. Tech. Papers, pp. 82-83,1998.

[0003] In [Document 1], there is disclosed a technique of stabilizingsense amplifier operation under condition of a reduced power supplyvoltage in a DRAM by applying a voltage having a potential differencewith respect to a final amplification voltage such as GND (e.g., anegative voltage lower than GND) to a source node of a CMOS senseamplifier. This technique is referred to as an “over-driving” schemesince there is provided a time interval during which the sense amplifieris driven by the voltage having a potential difference with respect tothe final amplification voltage on a bit line.

[0004] [Document 2]is mainly concerned with the technologies of dynamicrandom access memories (DRAMs), and on pages 161 to 167 thereof, a sensecircuit for amplifying a minuscule signal supplied fromamemory cell isexplained. In particular, pages 163 and 164 describe a method of drivinga plurality of sense amplifiers at high speed, under the section titleof “(2) Current-Distribution-Type Sense Amplifier Driving.” Morespecifically, according to this method, a sense amplifier driving powervoltage (equal to a final amplification voltage on a data line) issupplied in a meshed wiring arrangement, and a plurality of senseamplifiers are driven through one of driving MOSFETs disposeddistributively (e.g., four sense amplifiers are driven through onedriving MOSFET). [Document 3] and [Document 4] are cited in [Document 2]as the original technical literature proposing the above-mentionedmethod.

[0005] For the purpose of making it possible to implement anover-driving circuit for a large-capacity DRAM to be operated on a lowpower supply voltage, the inventors have examined some aspects ofpracticable arrangements of a sense amplifier and an over-driving drivecircuit therefor in the DRAM prior to preparing this patent application.

[0006]FIG. 25 shows an essential circuit part of the DRAM containing anover-driving drive circuit which has been examined by the inventorsprior to preparation of this patent application. The over-driving drivecircuit is designed to over-drive a P-side common source line CSP usinga voltage VDH higher than a high-level voltage “H” on a data line (VDL).In the over-driving drive circuit, an over-driving voltage VDH issupplied from a terminal of the P-side common source line CSP through aPMOS transistor QDP1 located thereat. In consideration of addition ofthe over-driving circuit, it is desirable to provide the over-drivingdrive circuit at a terminal of the CSP line as in the above-statedarrangement for reduction in circuit area.

[0007]FIG. 26 shows operating waveforms appearing on the common sourceline and data line in sense amplifier operation. It is herein assumedthat the data line and common source line are precharged with VDL/2before a sense amplifier starts amplification. Under condition that SP1is set to a low level to put QDP1 into conduction and the common sourceline CSP is supplied with the VDH, there are located SAn at the nearestposition to a VDH supply node and SA1 at the farthest positiontherefrom. An overdriving time period Tod representing a duration forwhich the QDP1 is put into conduction is set so that the “H” level sideof the data line will reach the VDL at high speed, not exceeding theVDL.

[0008]FIG. 26 (a) shows a case where the Tod is optimized with respectto the SAn which is located at the nearest position to a sense drivercorresponding to the VDH supply node, and FIG. 26 (b) shows a case wherethe Tod is optimized with respect to the SA1 which is located at thefarthest position therefrom. As shown in FIG. 26 (a), where the Tod isoptimized with respect to the nearest position, a voltage drop occurs onthe common source line due to a current supplied from the common sourceline to each SA in the initial period of sense operation. On the otherhand, at the farthest position, an OFF state takes place before asufficiently highlevel of voltage (CSP (1)) is not reached, resulting ina sufficiently high effective gate voltage not being attained asrequired. That is to say, data lines (D1t, D1b) are put in a low-speedoperation state. By way of contrast, as shown in FIG. 26 (b), where theTod is optimized with respect to the farthest position (SA1), the effectof over-driving becomes too high at the nearest position, causing a dataline voltage to exceed the VDL. This results in an increase in powerconsumption. As mentioned above, the inventors have found that a voltagedrop due to resistance on a common source line causes a decrease insense operation speed or an increase in power consumption, depending onthe position of each sense amplifier.

[0009] While a current concentration to a common source line of senseamplifiers and an effect on voltage attained thereby are discussed in[Document 2] to [Document 4], no consideration is given to applicationto an over-driving circuit for the sense amplifiers therein.

[0010] It is therefore an object of the present invention to provide asemiconductor device in which non-uniformity in over-driving among aplurality of sense amplifiers is eliminated. Another object of thepresent invention is to provide a semiconductor device in which anincrease in layout area including a plurality of sense amplifiers isreduced while eliminating non-uniformity in over-driving.

DISCLOSURE OF THE INVENTION

[0011] In accordance with a typical aspect of the present invention, aplurality of drive switches for over-driving are distributively disposedalong a row of sense amplifiers and a plurality of drive switches forrestore operation are concentratively provided at one end of the row ofsense amplifiers. A potential for over-driving is preferably suppliedusing a meshed power line circuit.

[0012] According to another aspect of the present invention, a pluralityof high-side drive switches for a plurality of sense amplifiers arestructured using MISFETs of the same conduction type as that of low-sidedrive switches for common use of a gate signal. This makes it possibleto reduce a distributed-arrangement layout area including the driveswitches and sense amplifiers.

[0013] Further, where MISFETs having a low threshold voltage are used asthe sense amplifiers, it is preferable to control a common source nodepotential of the sense amplifiers for decreasing a leak current in anactive-standby state. An impedance-variable sense amplifier drive switchis applicable as an example of preferable means for controlling a commonsource node potential of the sense amplifiers being activated.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a diagram showing a sense amplifier circuit in a firstpreferred embodiment of the present invention;

[0015]FIG. 2 is a diagram of operating waveforms in the first preferredembodiment of the present invention;

[0016]FIG. 3 is a diagram showing an essential part of a sense amplifiercircuit in a second preferred embodiment of the present invention;

[0017]FIG. 4 is a diagram of operating waveforms in the second preferredembodiment of the present invention;

[0018]FIG. 5 is a diagram showing an essential part of a sense amplifiercircuit in a third preferred embodiment of the present invention;

[0019]FIG. 6 is a diagram of operating waveforms in the third preferredembodiment of the present invention;

[0020]FIG. 7 is a diagram showing a sense amplifier circuit in a fourthpreferred embodiment of the present invention;

[0021]FIG. 8 is a diagram of operating waveforms in the fourth preferredembodiment of the present invention;

[0022]FIG. 9 is a diagram showing an essential part of a sense amplifiercircuit in a fifth preferred embodiment of the present invention;

[0023]FIG. 10 is a diagram of operating waveforms in the fifth preferredembodiment of the present invention;

[0024]FIG. 11 is a diagram showing a preferred embodiment in anapplication of the present invention to an ordinary sense operationmethod;

[0025] FIGS. 12 (a) and (b) are diagrams showing layout schemes of thesense amplifier circuits in the fourth and fifth preferred embodiments;

[0026]FIG. 13 is a diagram showing an example of a cross-sectionalstructure taken along line A-A' in the layout schemes of the senseamplifier circuits in FIGS. 12 (a) and (b);

[0027] FIGS. 14 (a) and (b) are diagrams showing examples ofcross-sectional structures taken along line B-B′and line C-C′ in thelayout schemes of the sense amplifier circuits in FIGS. 12 (a) and (b);

[0028]FIG. 15 is a diagram showing a sense amplifier circuit in a sixthpreferred embodiment of the present invention;

[0029] FIGS. 16 (a) to (d) are diagrams showing examples of arrangementsof Zn indicated in FIG. 15;

[0030] FIGS. 17 (a) to (d) are diagrams showing examples of arrangementsof Zp indicated in FIG. 15;

[0031]FIG. 18 is a diagram of operating waveforms in an applicationwhere the Zn and Zp arrangements shown in FIG. 16 (c) and FIG. 17 (c)are used in the sense amplifier circuit in FIG. 15;

[0032]FIG. 19 is a diagram showing an example of a circuit arrangementin an application to a low-Vt sense amplifier;

[0033]FIG. 20 is a diagram showing leak current paths in anactive-standby state;

[0034]FIG. 21 is a diagram of operating waveforms in connection withFIG. 20;

[0035]FIG. 22 is an entire configuration diagram of a synchronousdynamic random access memory in an application of the present invention;

[0036]FIG. 23 is a diagram showing an arrangement in which a memoryarray is divided into sub-memory arrays;

[0037]FIG. 24 is a diagram showing a meshed power line circuit in asub-memory array;

[0038]FIG. 25 is a circuit diagram showing an essential part of a DRAMcontaining an over-driving drive circuit examined prior to preparationof this patent application; and

[0039] FIGS. 26 (a) and (b) are diagrams showing examples of operatingwaveforms on common source and data lines in sense amplifier operationin connection with FIG. 25.

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] The present invention will now be described in detail by way ofexample with reference to the accompanying drawings. It is to beunderstood that the present invention is not limited to the use ofspecific circuit elements in each block in the following preferredembodiments. Inmost applications, the circuit elements in each block maybe formed on a semiconductor substrate such as a monocrystal siliconsubstrate using a known semiconductor device fabrication technique,e.g., a CMOS (Complementary MOS transistor) integrated circuitfabrication technique. In the drawings, a non-arrowed circuit symbol ofMOSFET (Metal Oxide Semiconductor Field Effect Transistor) represents anN-type MOSFET (NMOS), and an arrowed circuit symbol of MOSFET representsa P-type MOSFET (PMOS). For the sake of simplicity, each MOSFET ishereinafter referred to just as a MOS. Note that the present inventionis not limited to a circuit comprising a field effect transistor havingan oxide insulation film sandwiched between a metal gate and asemiconductor layer. It is to be understood that the present inventionis applicable to a circuit comprising an ordinary type of FET such as aMISFET (Metal Insulator Semiconductor Field Effect Transistor).

[0041] <Embodiment 1>

[0042] Referring to FIG. 1, there is shown a detailed diagram of asub-memory array SMA in a dynamic random access memory. In the presentpreferred embodiment, an over-driving operation is performed on one ofP-side and N-side source nodes of a sense amplifier in the initialperiod of amplification. The present preferred embodiment ischaracterized in that a plurality of over-driving drive switches QDP1are distributively disposed in a sense amplifier area SAA for driving aP-side common source line CSP. Before proceeding to detailed descriptionof FIG. 1, the following explains the present invention in terms ofentire positional relationship in a memory device with reference toFIGS. 22 and 23.

[0043] Referring to FIG. 22, there is shown an entire block diagram of asynchronous DRAM (SDRAM) in accordance with the present invention. Eachcircuit block is operated with timing of an internal control signalgenerated by a timing signal generator circuit TG to which a controlsignal is input. The following control signals are input to the TG undertiming control of a clock signal CLK; a chip select signal /CS, rowaddress strobe signal /RAS, column address strobe signal /CAS, and writeenable signal /WE. A combination of one of these control signals and anaddress signal is referred to as a command signal. A clock enable signalCKE is used to determine whether or not to enable the clock signal. Aninput/output mask signal DQM is used to control a data input/outputbuffer I/OB for masking input/output data on input/output terminals(DQO, . . . DQn).

[0044] The SDRAM employs an address multiplex method in which row andcolumn addresses are input from address input terminals (A0, A1, . . .An) in time-division multiplexing. A row address input to a row addressbuffer XAB is decoded by a row decoder X-DEC to select a particular wordline in a memory array MA0, and accordingly a memory cell for one wordis selected. Then, when a column address is input to a column addressbuffer YAB, the column address is decoded by a column address decoderY-DEC to perform further memory cell section for reading or writing. Inmost cases, the SDRAM is provided with a plurality of memory arrays (ormemory banks) that are specified with respective bank addresses. In FIG.22, only one memory array MA0 (BANKO) is shown as a representativearray.

[0045] Internal power voltages generated by a voltage generator circuitVG of the SDRAM shown in FIG. 22 are described below. In the presentpreferred embodiment, a single power supply method is employed in whichVCC (2.5 V) is supplied from an external circuit with reference to VSS(0 V). An internal power voltage having the highest potential is VPP(3.0 V), which is produced by a voltage boosting circuit containing acharge pump circuit, and the VPP thus produced is supplied to a wordline drive circuit and other circuits. VDH (2.5 =VCC) is used as a powervoltage for operating peripheral circuits such as the XAB, YAB, IOB andX-DEC. VDL (1.5 V) and VDBH (0 V =VSS) are supplied to a sense amplifierfor determining a data line restore potential, which will be describedlater. VDL is produced by a voltage down-converter circuit (voltagelimiter). Since a half-precharge method is adopted in the presentpreferred embodiment, VDL/2 (0.75 V) to be supplied to a circuit such asa data line in a standby state is produced from the VDL. The VDL/2 isalso used as a plate potential VPL for a memory cell. VBB (−0.75 V),used as a substrate potential for biasing a back gate of NMOS to thelowest circuit potential, is produced by the voltage boosting circuitcontaining the charge pump circuit.

[0046] Referring to FIG. 23, there is shown a detailed internalconfiguration of the memory array MA0 indicated in FIG. 22. The MA0comprises sub-memory arrays SMAll to SMAnm arranged in a matrix. The MAOis arranged in a hierarchical word line structure (divided word linestructure), though the present invention is not limited thereto. On oneside of the MAO, there is disposed a row of main word drivers MWD. Mainword lines connected with the MWD are provided in an upper metal wiringlayer so that they are extended across a plurality of sub-memory arrays(e.g., from SMAll to SMAnl). For column-wise selection, a common Ydecoder method is employed in which a plurality of column select lines(YS lines) from the column decoder Y-DEC are extended across a pluralityof sub-memory arrays (e.g., from SMAlm to SMAll). At the left and rightends of SMAll to SMAlm in the MAO shown in FIG. 23, there are provided aleft end area LEA and a right end area REA used for termination of thesub-memory arrays. The LEA and REA are arranged in somewhat modifiedforms of SAA and XA. This formation is made in consideration of mattermination since a shared sense method is employed in which senseamplifiers are provided in an alternate arrangement structure.

[0047] As shown in the enlarged view in FIG. 23, the inside of eachsub-memory array is divided into a memory cell area MCA, a senseamplifier area SAA, a sub word driver area SWDA, and a cross area XA. Inthis layout scheme, the MCA is formed in a square shape having first andsecond sides meeting each other at a corner common thereto, the SAA isformed in a rectangular shape along the first side of the MCA, and theSWDA is formed in a rectangular shape along the second side of the MCA.The XA is an area which is located outside the common corner to thefirst and second sides of the MCA and enclosed by the SAA and SWDA.

[0048]FIG. 1 shows a detailed diagram of the sub-memory arraycorresponding to the enlarged view in FIG. 23. In the memory cell areaMCA, a plurality of data line pairs D1t, D1b . . . Dnt, Dnb are arrangedto intersect a plurality of word lines WL for an array of memory cells,and each dynamic memory cell MC is connected at a predetermined point ofintersection. The MC comprises a capacitor for storing data and a MOStransistor, which is of an N type in the present preferred embodiment.While the data lines and memory cells are formed in a so-calledtwo-cross-points array structure (folded data line structure) in thepresent preferred embodiment, it is to be understood that the inventionis not limited thereto and may also be applicable to a one-cross-pointarray structure (open data line structure).

[0049] In the sub word driver area SWDA, a plurality of sub word driversSWD are provided respectively for a plurality of the word lines. The subword driver is activated by logical-ORing a signal on the main word linedescribed with reference to FIG. 23 and a control signal on an FX driverFXD, which is provided in the cross area XA (not shown in FIG. 1). Wherea word shunt structure is employed instead of the hierarchical word linestructure, the SWDA is provided with a lining word line, which is formedof a metallic material such as AL in an upper layer in lieu of the subword driver. In the SWDA, a through hole and a contact are also providedfor connection between a gate in a lower polysilicon layer and a commonword line. In this arrangement, the SWDA may be referred to as a wordshunt area.

[0050] The description of the sense amplifier area SAA is given below.In the SAA, elements such as a left/right shared switch SHR, a prechargecircuit PC, a sense amplifier SA1 and a column switch IOG are providedfor each pair of data lines (D1t, D1b). In practice of the presentinvention, 512 to 2048 pairs of data lines would be provided per memorycell area MCA. On this assumption, 256 to 1024 sense amplifiers aredisposed in each SAA, i.e., the number of sense amplifiers is half thenumber of data line pairs because of an alternate arrangement structureof the sense amplifiers. The shared switch is provided as a changeoverelement for common use of the sense amplifier SA1 to memory cell areason the left and right sides thereof. In the present preferredembodiment, the shared switch comprises an NMOS transistor, and gatecontrol signals SHRL and SHRR thereof have a potential VPP, VDH or VDLduring a period of data line precharge. For example, when access is madeto the left-side memory cell area, a condition “SHRL=VPP or VDH” or acondition “SHRR=VDBH” is taken, i.e., either one of SHRL and SHRR is putinto conduction without a decrease in NMOS threshold voltage. The PC isused to supply VDL/2 to each data line pair during a data line prechargeperiod according to a control signal PCS. The column switch IOG is usedto set up connection between a data line pair selected by a columnselect signal YS of the column decoder and a pair of common input/outputlines IOt and IOb for forming a data input/output path extending to anexternal circuit.

[0051] The sense amplifier SA is a latch-type amplifier circuit havingtwo CMOS inverters cross-coupled. More specifically, the sense amplifierSA comprises a source in common connection, a PMOS pair of cross-coupledgate and drain, and an NMOS pair of cross-coupled gate and drain. Thesources of the PMOS and NMOS pairs are connected to P-side and N-sidecommon source lines CSP and CSN respectively in common. For operation ofan over-driving type of sense amplifier, it is required to provide arestore potential and an over-driving potential. The restore potentialis a power supply potential used to determined high and low levels oneach data line at the time of final amplification. The term “restorepotential” is used since it is equal to a potential for memory cellre-write operation. The VDL corresponds to a high-side restorepotential, and the VDBH corresponds to a low-side restore potential. Inthe present preferred embodiment, a high-side over-driving potentialonly is supplied, i.e., a condition “VDH (>VDL)” is provided.

[0052] On the P side of each sense amplifier, a first power line forsupplying the high-side over-driving potential VDH is arranged inparallel with the P-side common source line CSP. A plurality of switchesQDP1 are distributively provided between the first power line and theP-side common source line CSP. In the example shown in FIG. 1, there isprovided one PMOS per sense amplifier. On the other hand, the high-siderestore potential VDL is not provided in the SAA. Using switches QDP12concentratively disposed in the cross area XA, the VDL is supplied fromone end of the P-side common source line CSP. A precharge circuit CSPCon the common source line is designed to perform short-circuiting forprecharge and VDL/2 compensation for leakage through the use of each endof the CSP and CSN disposed in the cross area XA.

[0053] On the N side of each sense amplifier, a second power line forsupplying the low-side restore potential VDBH is arranged in parallelwith the N-side common source line CSN. A plurality of switches QDN1 aredistributively provided between the second power line and the N-sidecommon source line CSN. In the example shown in FIG. 1, there isprovided one NMOS per sense amplifier in a fashion that a pair of QDN1and QDP1 is formed.

[0054] In the SAA, each sense amplifier PMOS pair and each over-drivingswitch MOS (QDP1) are formed in a common N-type well on a P-typesubstrate though the invention is not limited thereto. As a P-sidesubstrate bias, the VDH is applied to the N-type well. That is to say,the back gate of each PMOS is biased to the VDH which is equal to anover-driving potential. In a modified embodiment, the back gate of eachPMOS may also be arranged so that biasing to the VPP is made. In thesame manner, each sense amplifier NMOS pair and each QDN1 are formed ina common P-type doped semiconductor region (in a P-type substrate or ina triple well formed on a P-type substrate), and as an N-side substratebias, the VDBH or VBB is applied to the P-type doped semiconductorregion.

[0055] Referring to FIG. 24, there is shown an arrangement of wiring forfeeding the potentials VDH and VDBH used as power supply voltages inFIG. 1. The VDH and VDBH are supplied through a meshed power linecircuit having a low line impedance shown in FIG. 24. Vertical powerlines in FIG. 24 are formed in a second metal wiring layer M2 (made of amaterial such as aluminum). In the memory cell area MCA, VDH and VDBHsupply lines are arranged among and in parallel with main word linesMWL. In practice of the present invention, one main word line MWL wouldbe provided per four or eight word lines, for example. In the senseamplifier area SAA, VDH and VDBH supply lines are also arranged inparallel with the main word lines MWL. The VDH and VDBH supply lines inthe wiring layer M2 correspond to the first and second power linesdescribed with reference to FIG. 1.

[0056] On the other hand, horizontal power lines in FIG. 24 are formedin a third metal wiring layer M3 (made of a material such as aluminum)which is located at an upper position with respect to the wiring layerM2. Column select lines YS are arranged across the memory cell area MCAand the sense amplifier area SAA. One column select line YS is providedper four data lines, for example. In the memory cell area MCA and thesense amplifier area SAA, VDH and VDBH supply lines are arranged amongand in parallel with the column select lines. The VDH and VDBH supplylines, i.e., the VDH and VDBH power lines in the wiring layers M2 and M3are connected respectively at intersection points using through-holecontacts TH2 formed between the wiring layers M2 and M3. Theintersecting power lines in the wiring layers M2 and M3, i.e., the VDHand VDBH lines formed in a meshed power line structure have a lowimpedance.

[0057] Referring to FIG. 2, there is shown an operation timing chart ofthe sub-memory array diagrammed in FIG. 1. On input of a low-activecommand to the SDRAM, memory cells connected with a particular main wordline in a particular bank are read through sense amplifierssimultaneously for amplification. Thereafter, when a precharge commandis input, the memory cells are deselected to set up a precharge statewhich is a waiting state for the next read operation. The waveformsshown in FIG. 2 indicate operations to be performed in the sub-memoryarray diagrammed in FIG. 1 from a point of time the low-active commandis input until a point of time the precharge command is input.

[0058] When a precharge control signal PCS is made active on the dataline and the common source line, VDL/2 precharging of the data line andthe common source line is stopped. Then, one of plural word lines WL isselected to cause a transition from a VWL level (VWL=VSS under normalcondition) to aVPP level. Amemory cell MC is thus selected, and the VPPis applied to the NMOS transistor gate thereof for activation. Then, acharge accumulated in a capacitor for storing data is read out onto datalines D1t . . . Dnt connected with the memory cell MC. A charge in thecell causes a minuscule voltage difference between a pair of the datalines. When data in the cell is “H”, the voltage level of D1t becomesapproximately 100 mV higher than that of D1b. In this example, it isassumed that “H” data is stored in the cell capacitor of the memory cellMC. In a situation where low-level data “L” is stored in the capacitorof the memory cell MC, the same sequence is performed except that alower potential is used.

[0059] At the start of sense operation after completion of the readingof cell data, an N-side common source drive control signal line SN isset from the VDBH to a level higher than the VDL to activate a QDN,thereby driving the CSN to make a transition from the VDL/2 to the VDBH.At the same time or after a lapse corresponding to the number of delaystages, a first P-side common source drive control signal line SP1 isset from the VPP to the VSS, for example, to activate the QDP1, therebydriving the CSP to make a transition from the VDL/2 to the VDH. At thisstep, the VDH is supplied through the low-impedance meshed power linecircuit and the distributively arranged switches QDP1 as described withreference to FIGS. 1 and 2. Therefore, the sense amplifiers SA1 to SAnare activated simultaneously with almost the same timing, thus making itpossible to suppress variation in over-driving for the SA1 to SAn. Thisalso enables high-speed driving of the common source lines CSP and CSN.Further, in over-driving drive operation, since a source-drain voltageand a gate-source voltage of a PMOS transistor in the SA become higherthan the VDL/2, it is possible to amplify a minuscule voltage differenceΔV on a pair of data lines at high speed.

[0060] Under condition that amplification on the highlevel-side dataline Dlt is not yet completed, a sense amplifier over-driving period isset to a time period Tp1 to be taken for a potential on the data lineDlt to become close to the VDL. In consideration of power consumptionand other factors, it is preferable to stop over-driving before thepotential on the data line becomes higher than the VDL. After a lapse ofthe time period Tp1, the SP1 is set from the VSS to a level higher thanthe VDH, e.g., the VPP. Then, a second P-side common source drivecontrol signal line SP2 is set from the VPP to the VSS, for example,thereby activating the QDP2 to set the CSP to the VDL. Thus, thehigh-level-side data line is maintained at the VDL.

[0061] After input of the precharge command, the following operation isperformed. The selected word line WL is set from the VPP to the VWL.Thereafter, the SN is set from the VDL or VPP to the VDBH, and the CSNis disconnected from the VDBH. At almost the same time, the SP2 is setfrom the VSS to the VPP, and the CSP is disconnected from the VDL. TheCSN and CSP, which have thus been disconnected from the power supply,and data line pairs D1t, D1b . . . Dnt, Dnb are precharged to the VDL/2according to the precharge control signal PCS.

[0062] In the present preferred embodiment, the following advantageouseffects are provided: (1) At the time of over-driving, a charge currentis output from the over-driving power supply to each data line, and thecharge current thus output is supplied through the meshed power linecircuit and the plural switches QDP1 which are distributively disposedin the vicinity thereof. Thus, a current concentration to a particularsense amplifier and a particular source line CSP can be circumvented toallow over-driving for any sense amplifiers SA1 to SAn with an equallevel of over-driving voltage (VDH). (2) An over-driving period can beset according to a time period that the QDP1 is activated with a gatesignal on the SP1, thereby allowing uniform operation among the senseamplifiers SA1 and SAn. Thus, it is possible to reduce a difference inover-driving amplitude and period with respect to far and near positionsof the sense amplifiers arranged distributively. (3) Since a chargecurrent from each data line to terminal VDBH is delivered to the meshedpower line circuit VDBH on each array through a multiplicity of theswitches QDN, a current concentration to a particular sense driver andCSN can be prevented.

[0063] In the present preferred embodiment, either one or both of theQDP1 and QDP2 may be formed in an NMOS transistor structure. In thiscase, it is required to invert control signal logic employed for PMOStransistor formation. Where NMOS transistors are used as the QDP1 andQDP2, a gate-source voltage becomes negative in an inactive state,thereby providing an advantage that a leak current from the VDH/VDL tothe CSP can be reduced.

[0064] While one MOS switch QDP1 and one MOS switch QDN1 are disposedper sense amplifier in the present preferred embodiment, there may alsobe provided such a modified arrangement that one MOS switch QDP1 and oneMOS switch QDN1 are disposed per two, four or eight sense amplifiers.Further, in another modified arrangement, the MOS switches QDP1 and QDN1may be structured as one MOS switch having a long gate forsingle-row-form connection instead of dividing a diffusion layer inwhich channels are structured along the sense amplifiers. Since afeature of the present invention is to use the MOS switchesdistributively arranged in the SAA area for the purpose of over-driving,it is not important whether a channel width is divided into sections ornot.

[0065] Embodiment 2

[0066] Referring to FIG. 3, there is shown a configuration of a senseamplifier circuit in a second preferred embodiment of the presentinvention. An essential part of the sense amplifier circuit isdiagrammed in FIG. 3, and the other parts thereof are the same as thosein the first preferred embodiment. In the second preferred embodiment,N-side over-driving is also provided in addition to P-side over-drivingin the scheme shown in FIG. 1. The second preferred embodiment isdifferent from the first preferred embodiment in that aconcentrative-type switch QDN2 is added at one end of the N-side commonsource line CSN in the cross area XA. Through the QDN2, the low-siderestore potential VDBH (VSS under normal condition) on each data line isapplied. In the meshed power line circuit, a voltage VDBL lower than theVDBH is supplied as an N-side over-driving power voltage instead of theVDBH. The VDBL is applied to the N-side common source line through thedistributively arranged switches QDN1. Since the overdriving of theN-side common source line is performed with the VDBL, each back gate ofthe sense amplifier NMOS pair and QDN1 is biased to a voltage equal toor lower than the VDBL. In the above-mentioned arrangement shown in FIG.3, the VDL and VDBH are used as high-side and low-side restorepotentials respectively, and the VDH (>VDL) and VDBL (<VDBH) are used ashigh-side and low-side over-driving potentials respectively.

[0067] Referring to FIG. 4, there is shown a diagram of operatingwaveforms in the scheme presented in FIG. 3. As in the case of the firstpreferred embodiment, it is assumed that “H” data is stored in the cellcapacitor of the memory cell MC. Unlike the first preferred embodimentshown in FIG. 2, SN1 and SN2 are controlled because of addition ofN-side over-driving.

[0068] After completion of the reading of cell data, a voltage on theDlt becomes approximately 100 mV higher than that on the D1b. Then, theSN1 makes a level transition from the VDBL to the VDL or VPP, therebyactivating the QDN1. At the same time or after a lapse corresponding tothe number of delay stages, the SP1 is set from the VPP to the VSS toactivate the QDP1. Then, the CSN makes a transition from the VDL/2 tothe VDBL, and the CSP makes a transition from the VDL/2 to the VDH. Atthe start of these transitions of the CSN and CSP to the VDBL and VDHrespectively, the SA1 connected with a pair of the data lines Dlt andD1b is activated to amplify a minuscule voltage difference between thedata lines. At this step, the SA1 is activated with an amplitude largerthan a data-line amplitude VDL (VDH VDBL) through over-driving.Therefore, a source-drain voltage and a gate-source voltage of the NMOSand PMOS transistors constituting the SA1 are increased to enablehigh-speed operation. To prevent an increase in charge-discharge powerdue to excessive amplification, the QDN1 is activated under conditionthat amplification on the low-level-side data line to the VDBL is notyet completed; more specifically, the QDN1 is activated during a timeperiod Tn1 to be taken until a state not reaching a level lower than theVDBH persists. Similarly, the QDP1 is activated under condition thatamplification on the high-level-side data line to the VDH is not yetcompleted; more specifically, the QDP1 is activated during a time periodTp1 to be taken until a state not exceeding the VDL persists. Control ofactivation time is carried out by the SP1 and SN1. As in the firstpreferred embodiment, an over-driving period in the SAn is equal to thatin the SA1, and the Tn1 and Tp1 are set on the low level side and highlevel side respectively. Since an over-driving voltage at this step issupplied by the QDN1 and QDP1 located near the SAn, the VDBL and VDH areset on the low level side and high level side respectively in the samemanner as in the SA1.

[0069] After completion of over-driving operation, the SN2 is set fromthe VDBL to the VDL or VPP, and the CSN is set to the VDBH. Theactivation timing of the SN2 is so controlled that the QDN1 and QDN2 aremade active simultaneously to prevent connection between the VDBL andthe VDBH through the CSN. Thus, the low-level-side data line D1b ismaintained at the VDBH. Further, the SP2 is set from the VPP to the VSS,thereby setting the CSP to the VDL. The activation timing of the SP2 isso controlled that the QDP1 and QDP2 are made active simultaneously toprevent connection between the VDH and the VDL through the CSP. Thus,the high-level-side data line Dlt is maintained at the VDL. Finally, theword line goes low for restoration to a precharge state in the samemanner as in FIG. 2.

[0070] In the second preferred embodiment, the following advantageouseffects are provided: (1) Similarly to the first preferred embodiment,in over-driving on the high-level-side data line, an equal over-drivingvoltage and an equal over-driving period can be set for all the senseamplifiers SA, thereby making it possible to reduce a difference insense operation speed with respect to far and near positions of thesense amplifiers arranged distributively. (2) Further, unlike the firstpreferred embodiment, over-driving is performed on the low-levelsidedata line also, which allows shortening a sense operation time for usewith the same data line amplitude. Since over-driving on the low levelside is also performed as noted above, it is possible to circumvent apossible trouble due to a smaller data line amplitude, i.e., a decreasein operating voltage. (3) Still further, in over-driving on thelow-level-side data line, a current concentration to a particular sensedriver and a particular CSN in sense operation can be circumvented sincethere are provided a multiplicity of the switches QDN1 and the meshedpower line circuit on each array. Even during an overdrive period,common signal SN1 setting can be made for the sense amplifiers SA1 toSAn. Thus, it is possible to reduce a difference in over-drivingamplitude and period with respect to far and near positions of the senseamplifiers arranged distributively. (4) In the second preferredembodiment, just one MOS element is additionally provided in the crossarea, thereby resulting in no virtual increase in the size of the senseamplifier area.

[0071] Embodiment 3

[0072] Then, the following describes the configuration of a senseamplifier circuit in a third preferred embodiment with reference to FIG.5. The third preferred embodiment is a modification of the secondpreferred embodiment shown in FIG. 3, based on the configuration shownin FIG. 1. While the switches QDP2 and QDN2 for restore operation areconcentratively arranged in the cross area XA in the second preferredembodiment, there are distributively arranged switches QDP2 and QDN2 inthe sense amplifier area SAA in the third preferred embodiment. Thedistributed arrangement of the QDP2 and QDN2 and the meshed power linestructure of the VDL and VDBL are provided similarly to the firstpreferred embodiment shown in FIG. 1. FIG. 6 shows a diagram ofoperating waveforms in the scheme presented in FIG. 5. The operatingwaveforms in FIG. 6 are the same as those in FIG. 4.

[0073] In the third preferred embodiment, the following advantageouseffects are provided: (1) As in the second preferred embodiment,over-driving is performed on both the high-level-side and low-level-sidedata lines to realize high-speed sense operation. (2) An equalover-driving voltage and an equal over-driving period can be set for allthe sense amplifiers SA, thereby making it possible to reduce adifference in sense operation with respect to far and near positions ofthe sense amplifiers arranged distributively. (3) As compared with thesecond preferred embodiment, a current concentration to a particular CSNand a particular CSP can be circumvented even in restore operation sincethere are provided a multiplicity of the switches QDN2 and QDP2 in thesense amplifier area. (4) All the sense drivers are disposed in thesense amplifier area, thereby making it possible to simplify the layoutof elements other than the sense amplifiers.

[0074] Embodiment 4

[0075] Referring to FIG. 7, there is shown a configuration of a senseamplifier circuit in a fourth preferred embodiment of the presentinvention. The fourth preferred embodiment is also based on the firstpreferred embodiment. The fourth preferred embodiment is characterizedin that all the P-side and N-side over-driving switches MOS arestructured using transistors of the same conduction type, e.g., NMOStransistors as shown in FIG. 7, in that gate signals thereof are used incommon, and in that the P-side and N-side over-driving switches aredriven by a signal having a level sufficiently higher than theover-driving voltage VDH such as the VPP corresponding to a word-lineboosted level. Since the P-side over-driving switch of an NMOStransistor type is used, a voltage drop due to the P-side NMOStransistor can be prevented in the fourth preferred embodiment. Thefourth preferred embodiment is a modification of the second preferredembodiment shown in FIG. 3 in which the over-driving switches MOS aredistributively arranged. In the fourth preferred embodiment, one P-sideover-driving switch MOS QDP1 and one N-side over-driving switch MOS QDN1are disposed per four sense amplifiers in the sense amplifier area SAA.The gates of the QDN1 and QDP1 have a common connection to anover-driving control signal line SAE1. The high-side and low-sideover-driving potentials VDH and VDBL are supplied through the meshedpower line circuit similarly to the other preferred embodimentsdescribed in the foregoing. The restore potentials are supplied throughswitches QDP2 and QDN2 concentratively disposed in the cross area XA inthe same manner as in the scheme shown in FIG. 3.

[0076] Referring to FIGS. 12 (a) and (b), there are shown plan layoutviews of a sense amplifier circuit in the fourth preferred embodiment.In FIG. 12 (a), four pairs of data lines are diagrammed. For the sake ofsimplicity, a first metal wiring layer (metal 1, M1), transistor gates,gate wiring (FG), a diffusion layer, and NWEL only are shown. Adesignation SAN indicates an NMOS transistor part of the SA, and adesignation SAP indicates a PMOS transistor part thereof. The switchesQDN1 and QDP1 comprise NMOS elements having gates in a single-row formbetween the SAN and SAP. The fourth preferred embodiment ischaracterized in that the NMOS elements arranged in a single-row formare allocated to the QDN1 and QDP1 in an alternate fashion. In thisalternate arrangement, one control electrode SAE1 is used in common,thereby contributing to reduction in the size of the layout area. Whileone QDN1 and one QDP1 are disposed between the SAN and SAP per four dataline pairs in the layout scheme shown in FIG. 12 (a), it is to beunderstood that the present invention is not limited to thisdisposition. For example, in a modified form, one QDN1 and one QDP1 maybe disposed per eight or sixteen data line pairs. In consideration ofconnection with both the P-side and N-side common sources, it would bemost rational to arrange the QDN1 and QDP11 between the SAN and SAP inthe sense amplifier area. 'However, it will be appreciated that thepresent invention is not limited to this arrangement.

[0077]FIG. 12 (b) is a plan layout view of a sense amplifier circuitwherein the same layer Ml as that in FIG. 12 (a) is omitted and a secondmetal wiring layer (metal 2, M2) located above the M1 is added. In theM2, the P-side common source line CSP, power line VDBL for supplyingVDBL, power line VDH for supplying VDH, and N-side common source lineCSN are formed in succession. These four wiring lines are extended inthe direction in which the sense amplifiers are arranged in a single-rowform (in the extending direction of the word line). This arrangement ofthe four wiring lines is made for the purpose of reducing the size ofthe sense amplifier circuit layout area in the present preferredembodiment. The above arrangement is reflected in the circuit schemeshown in FIG. 7, i.e., FIG. 7 is a simplified diagram of the senseamplifier circuit layout. In FIG. 9 and the subsequent circuit schemesto be described later, concrete essential parts of respective circuitconfigurations are diagrammed in the same manner.

[0078] As to the channel width structures of the QDP1 and QDN1 shown inFIG. 12 (a), the channel width of the QDP1 is preferably equal to thatof the QDN1 (it is preferable to form NMOS elements having the samesize). Thus, the sense amplifier SAN turns on before the SAP turns on.In the SAN comprising the NMOS transistor which has smaller fluctuationin Vt due to process variation than the PMOS, differential amplificationis started using a minuscule voltage difference, thereby making itpossible to ensure high accuracy in differential amplification. Both theQDP1 and QDN1 are of an NMOS type, and each of them is formed in aP-type well (in a P-type substrate in the present preferred embodiment).The P-type well is supplied with the lowest potential (e.g., VDBL in thepresent preferred embodiment). Therefore, a relatively higher substratebias is applied to the QDP1 having a larger potential, and the thresholdvoltage of the QDP1 becomes higher than that of the QDN1. Inconsequence, the QDN1 having a lower threshold voltage is more likely toturn on, causing the SAN to be driven first.

[0079]FIG. 13 shows a cross-sectional view taken along line A-A′ inFIGS. 12 (a) and (b), and FIGS. (a) and (b) show cross-sectional viewstaken along lines B-B′ and C-C′ respectively. In these cross-sectionalviews, a designation SGI (shallow groove isolation) indicates aninsulating part for isolation of each diffusion layer (N+, P+in thefigures), which is formed by embedding a material such as Si oxide intoa shallow groove in the substrate. A designation CNT indicates a contacthole for connection between the metal layer 1 (Ml in the figures) andthe diffusion layer or FG. A designation TH1 indicates a contact holefor connection between the Ml and the metal layer,2 (M2 in the figures),and a designation TH2 indicates a contact hole for connection betweenthe M2 and the metal layer 3 (M3 in figures). As shown in FIG. 14 (a),the CSN and the drain of the QDN1 are connected through the M3. As canbe seen from this figure, merely electrical connection may be arrangedbetween the CSN and the drain of the QDN1. In the present preferredembodiment, the M3 is used for connection between the CSN and the drainof the QDN1 for the purpose of providing equal resistance between thedrain of the QDN1 and each source of two NMOS elements constituting theSAN. The diffusion layer P+is also connected so that the sourcepotentials of the two NMOS elements constituting the SAN will be equalto each other. Thus, the circuit layout is designed to prevent anunbalanced condition between the two NMOS elements constituting the SAN.The CSN and CSP are formed in the M2 on the SAN and SAP, respectively.Similarly to each source of the two NMOS elements, as shown in FIG. 14(b), the CSP and the source of the QDP1 (NMOS source of the QDP1) areconnected through the M3. Each source of two PMOS elements constitutingthe SAP and the source of the QDP1 are also connected in a fashionsimilar to that mentioned above.

[0080] The following describes operations in the fourth preferredembodiment with reference to FIG. 8 which shows an operating waveformdiagram. After completion of precharging of the data line, a minusculevoltage difference is produced on the data line in the same manner asthat in the foregoing preferred embodiments. Data stored in each cell isread out onto the data line, and then the SAE1 is set from the VDBL tothe VPP, thus activating the QDN1 and QDP1. Then, the CSN starts makinga transition from the VDL/2 to the VDBL, and the CSP start making atransition from the VDL/2 to the VDH. At this point in time, thethreshold voltage Vt of the QDP1 is higher than that of the QDN1 due toan effect of substrate biasing even where the QDP1 and QDP1 arestructured using NMOS transistors having the same physical constant.Therefore, even if the same voltage is applied as a gate signal, theQDN1 is driven before the QDP1. To prevent an increase in currentconsumption which may result from an excessive amplitude ofamplification on the data line, the QDN1 and QDP1 are made active by theSAE1 just for a period of time Tnp that a voltage on the low-level-sidedata line becomes less than the VDBH or a voltage on the high-level-sidedata line does not exceed the VDL. Since an over-driving period in theSAn is determined by the gate signal SAE1, the over-driving period inthe SAn is equal to that in the SA1, i.e., it is equal to the Tnp.Thereafter, the SAE1 is set from the VPP to the VDBL to completeover-driving operation. At the same time that the SAE1 is set to theVDBL, the SN2 is set from the VDBL to the VDL or VPP to activate theQDN2. Thus, the CSN is set to the VDBH, and the low-level-side data lineD1b is restored to the VDBH. Similarly, after the SAE1 is set to theVDBL, the SP2 is set from the VPP to the VSS to activate the QDP2. Thus,the CSP is set to the VDL, thereby restoring the high-level-side dataline Dlt to the VDL. Finally, the word line goes low for restoration toa precharge state in the same manner as in the foregoing preferredembodiments.

[0081] In the fourth preferred embodiment, the following advantageouseffects are provided: (1) In the circuit 25 layout, the QDP1 comprisingan NMOS transistor is used, and the QDN1 and QDP1 are arranged in asingle-row form on the sense amplifier area. Thus, the gate controlsignal can be used in common with the QDN1. As compared with the first,second and third preferred embodiments where the NMOS and PMOStransistors are provided, i.e., the NMOS and PMOS transistors arearranged in a two-row form, the single row-form arrangement of the QDN1and QDP1 requires a smaller space for the sense amplifier area. (2)Further, as compared with the circuit scheme shown in FIG. 3 whereover-driving is performed on both the CSN and CSP, one over-drivingcontrol signal line can be eliminated in the fourth preferredembodiment, contributing to reduction in the size of the control signalcircuit. (3) Since both the QDP1 and QDN1 comprising NMOS transistorsare biased with the back gate using the same voltage, the QDN1 is drivenbefore the QDP1 when the SAE1 signal is input at the start of senseoperation. Therefore, differential amplification can be started using aminuscule voltage difference through the NMOS transistor which hassmaller fluctuation in Vt due to process variation than the PMOS,thereby making it possible to ensure high accuracy in differentialamplification. (4) Since the QDP1 of an NMOS transistor type is used, agate-source voltage of the QDP1 becomes negative when the SAE1 signal isin the VDBL state. Therefore, while the QDP1 is inactive, it is possibleto suppress current leakage from the VDH to the VDL/b 2. (5) As in thefirst, second and third preferred embodiments, an equal over-drivingvoltage and an equal over-driving period can be set for all the senseamplifiers SA, thereby allowing reduction in difference in senseoperation with respect to far and near positions of the sense amplifiersarranged distributively.

[0082] While over-driving is performed on both P and N sides in thefourth preferred embodiment, the low-level restore potential VDBH may beused in lieu of the VDBL on the power line in the scheme shown in FIG. 8in a case where over-driving on a single side satisfies requirements interms of relationship with power supply voltage. This arrangementeliminates the need for providing a large capacity negative powergenerator circuit for supplying the VDBL, thereby contributingadvantageous reduction in chip area. Further, since the kinds of powerlines for the sense amplifier circuit can be decreased consequently,there is provided an advantage that the meshed power line circuit issimplified.

[0083] In [Document 5], there is disclosed a circuit configuration inwhich NMOS transistors are used for high-level and low-level restorevoltages to each CMOS sense amplifier in a DRAM. However, the circuitconfiguration disclosed in [Document 5] is based on the premise that apower voltage VCC is supplied as a word line drive voltage. On thispremise, a threshold voltage Vt of each P-side switch NMOS isintentionally reduced to decrease the high-level restore voltage on adata line to a level of VCC-Vt. Therefore, the object of the circuitconfiguration disclosed in [Document 5] is different from that of thepresent invention. Further, [Document 5] gives no description todistributed arrangement of MOS switches and over-driving operation.

[0084] Embodiment 5

[0085] Referring to FIG. 9, there is shown a configuration of a senseamplifier circuit in a fifth preferred embodiment of the presentinvention. The fifth preferred embodiment is characterized in that therestore switches MOS of an NMOS type in FIG. 7 are distributivelyarranged in the sense amplifier area SAA, and in that the controlsignals are used in common in the same manner as in FIG. 7. The P-sideand N-side over-driving switches NMOS QDP1 and QDN1 are structured as inFIG. 7. Unlike the circuit scheme shown in FIG. 7, the restore switchesQDP2 and QDN2 are also distributively arranged in the sense amplifierarea in the fifth preferred embodiment. The gates of the QDP2 and QDN2are controlled through a common control line SAE2. Further, the high andlow restore potentials, i.e., the VDL and VDBH are supplied through themeshed power line circuit described with reference to FIG. 24. One QDP2and one QDN2 are provided per four sense amplifiers. The QDN1, QDP1,QDN2 and QDP2, which are NMOS transistors having two rows of gates, aredisposed in a single-row form parallel to the SAN and SAP rows.

[0086] It is to be understood that the present invention is not limitedto correspondence relationship among the number of sense amplifiers, thenumber of over-driving switches MOS, and the number of restore switchesMOS used in the fifth preferred embodiment. For example, in a modifiedarrangement, each one of QDP1, QDP2, QDN1 and QDN2 may be provided pereight sense amplifiers. Further, since charging on the common sourceline is mainly performed by the over-driving switch, the restore switchmay have a relatively small driving capacity. This arrangement is morerational than a configuration in which the number of over-drivingswitches QDP1 and QDN1 is larger than the number of switches QDP2 andQDN2. That is to say, in general, there may be provided such anarrangement that conductance of all the over-driving switches MOS ishigher than that of all the restore switches MOS.

[0087] The following describes operations in the fifth preferredembodiment with reference to FIG. 10 which shows an operating waveformdiagram. After completion of precharging, the SAE1 is set to the VPP tostart overdriving in the same manner as in FIG. B. To prevent anincrease in current consumption due to excessive sensing, the QDN1 andQDP1 are made active by the SAE1 just for a period of time Tnp that avoltage on the low-level-side data line becomes less than the VDBH or avoltage on the high-level-side data line does not exceed the BDL. Sincean over-driving period in the SAn is determine by the gate signal SAE1,the over-driving period in the SAn is equal to that in the SA1, i.e., itis equal to the Tnp. Thereafter, the SAE2 is set from the VDBL to theVPP, the CSN is set to the VDBH, and the low-level-side data line D1b isrestored to the VDBH. At the same time, the CSP is set to the VDL, andthe high-level-side data line D1t is restored to the VDL. For the SAE2,the QDN1, QDN2, QDP1 and QDP2 are activated simultaneously, and thepower potentials VDBL, VDBH, VDH and VDL are controlled not to incurshort circuiting through the CSN and CSP.

[0088] In the fifth preferred embodiment, the following advantageouseffects are provided: (1) In the sense amplifier circuit layout, thesense drivers of an NMOS type are arranged in a two-row form. Althoughthis results in an increase in the size of the sense amplifier layoutarea in comparison with the fourth preferred embodiment, there is noneed for providing the sense drivers in other than the sense amplifierarea. Therefore, the circuit layouts of other than the sense amplifierarea can be simplified. (2) As compared with the second preferredembodiment in which over-driving is performed on both the high-levelsideand low-level-side data lines, the number of sense amplifier controlsignals can be decreased by two signals. This leads to reduction in thesize of the control signal circuit. (3) As in the first to fourthpreferred embodiments, an equal over-driving voltage and an equalover-driving period can be set for all the sense amplifiers SA, therebyallowing reduction in difference in sense operation with respect to farand near positions of the sense amplifiers arranged distributively. (4)Since the QDP1 and QDP2 of an NMOS transistor type are used, agate-source voltage VGS of each of the QDP1 and QDP2 becomes less than 0V (VGS <0 V) in a standby state. Therefore, current leakage from the VDHand VDL to the VDL/2 can be suppressed.

[0089] There may also be provided such a modified arrangement that theVDBL is equal to the VDBH though the degree of improvement in senseoperation speed is decreased. This arrangement eliminates the need forproviding a large-capacity negative power supply circuit, therebycontributing advantageous reduction in chip area. Further, since justthree kinds of power lines are required for the sense amplifiers, thepower line circuit on the memory array can be simplified advantageously.

[0090] The present invention is also applicable to a sense amplifierconfiguration without using an over-driving scheme. FIG. 11 shows apreferred embodiment in an application of the present invention to anordinary sense operation method. Since over-driving is not used in thissense amplifier configuration, a PMOS-to-SAP substrate potential is setat the VDL for each sense amplifier. Further, there is no need forproviding the sense drivers in other than the sense amplifier area.Therefore, the circuit layouts of other than the sense amplifier areacan be simplified advantageously.

[0091] While all the sense amplifiers in FIGS. 7 to 11 comprise NMOStransistors, there may be provided such a modification that PMOStransistors are used as the sense amplifiers.

[0092] In the preferred embodiments mentioned above, either low or highVt may be employed for the sense drivers and SA transistors. Wherelow-Vt transistors are used, sense amplifier operation can be performedat higher speed than that in the use of high-Vt transistors. Where thehigh-Vt transistors are used, current leakage in an SA data holdingstate can be reduced to decrease power consumption. In contrast, wherethe low-Vt transistors are used, it is possible to reduce currentleakage by the method to be described later. Further, by using thehigh-Vt transistors as the sense drivers, current leakage between thesense amplifier power supply and the VDL/2 can be reduced in a standbystate.

[0093] In the first to fifth preferred embodiments, it is preferable toprovide the following voltage relationship. As to VWL and VPP inamplitude on word line WL, VDBH and VDL in amplitude on data line, VDBLand VDH used as power voltages for initial sense operation, andsubstrate potential VBB, the voltage relationship indicated below may beset up for decreasing the number of internal power sources:

VBB=VDBL (−0.75 V)<VWL=VDBH=VSS (0 V) <VDL (1.5 V)<VDH (2.5 V)<VPP (3 V)

[0094] Under condition that VBB<VDBL (−0.5 V), variation in memory cellsubstrate bias can be suppressed advantageously though the number ofpower sources is increased. Further, under condition that VDH=VPP (3 V),each sense amplifier can be activated with higher power supply.

[0095] For power voltage setting, a negative word method in which aword-line standby voltage level is negative may be employed as reportedin [Document 6]. For applying the negative word method to the preferredembodiments mentioned above, it is required to provide the followingcondition:

VBB=VDBL=VWL (−0.75 V)<VDBH=VSS (0 V)<VDL (1.5 V)<VDH=VPP (2.25 V)

[0096] The use of the negative word method provides an advantageouseffect that the number of internal power supply levels can be reduced.Further, under condition that VBB<VDBL<VWL, VBB<VWL<VDBL, orVBB<VDBL=VWL in an arrangement that VBB is provided separately fromother power sources, variation in VBB used as a memory cell arraysubstrate bias can be suppressed though the number of power supplylevels is increased. Thus, the data holding characteristic of each cellcan be improved advantageously.

[0097] In the schemes described above, it is preferable to use anexternal power voltage VCC as the VDH. In a modified arrangement, avoltage level stepped up by a voltage boosting circuit or a voltagelevel stepped down by a voltage down-converter circuit may be used asthe VDH.

[0098] Embodiment 6

[0099] While the over-driving methods have been discussed in theforegoing preferred embodiments, it is considered that reduction inthreshold voltage Vt for each sense amplifier is required in a situationwhere the power supply voltage is decreased. By over-driving each senseamplifier comprising a low-threshold-voltage MOS transistor, theamplitude on an operable data line could be further decreased forreduction in power consumption. However, since the use of alow-threshold-voltage MOS transistor increases a sub-threshold currentto result in an increase in current consumption in a waiting state,there would occur a problem in consistency with an active-standby statein the SDRAM. Therefore, a method for reducing a sub-threshold currentunder condition that data is latched by the sense amplifier comprising alow-threshold-voltage MOS transistor is disclosed herein as a fifthpreferred embodiment.

[0100] Referring to FIG. 20, there is shown a sub-threshold current ofthe sense amplifier when a signal from each data line is amplified andlatched by the sense amplifier. In the SDRAM, a particular word ofmemory cell data is amplified and latched by the sense amplifier using alow-active command. This operating condition is referred to as anactive-standby state. In the active-standby state, data is held by thesense amplifier in advance for realizing high-speed access. As shown inFIG. 20, under condition that data is held by each sense amplifier, asub-threshold current “i” flows per sense amplifier. As to the senseamplifier CMOS connected in series between the VDL and VDBH, either PMOSor NMOS transistor has a gate-source voltage of 0 V in an OFF state.However, if the threshold voltage is low, a complete OFF state is notset to produce a flow of sub-threshold current to be considered.Therefore, as shown in the waveform diagram in FIG. 21, a leak current“ni” is fed from the power supplyVDL to the VDBH resultantly. In aninstance where 64k sense amplifiers comprising transistors having athreshold voltage Vt of 0.1 V are set to the active-standby state, asub-threshold current of approximately 3 mA is fed to prevent reductionin power consumption. Further, in an instance where the thresholdvoltage Vt of each transistor is decreased by 0.1 V, the sub-thresholdcurrent is increased approximately ten times. Therefore, in a case wherethere is variation in threshold voltage Vt among fabricated transistorsor in an application where the transistors are used at a hightemperature level that the threshold voltage Vt tends to decrease, thesub-threshold current in low-Vt MOS gives rise to a considerableproblem.

[0101] Referring to FIG. 15, there is shown a circuit scheme in which amethod of sub-threshold current reduction in the active-standby state isapplied to the over-driving SA configuration according to the presentinvention. The sixth preferred embodiment is based on the foregoingpreferred embodiments, and a similar circuit structure is used therein.The circuit scheme shown in FIG. 15 will be easier to understand throughexamination in comparison with that in FIG. 3 in particular.

[0102] First, the following explains a principle of leak currentreduction in a sense amplifier. After cell data is amplified by thesense amplifier SA, the CSN is set at the VDBH and the CSP is set at theVDL. At this step, a value of MOS substrate bias contained in the SA isequal to a design value, e.g., in an NMOS transistor, the substrate biashas a value of VBB. Then, when the CSN makes a level transition from theVDBH to VDBH′ (>VDBH), the substrate bias is increased by (VDBH′-VDBH).Thus, the effect of the substrate bias increases the threshold voltageVt of the NMOS transistor. More specifically, under condition that thegate and source of the NMOS transistor are short-circuited, a constantvoltage (substrate voltage) is applied to the back gate and a sourcepotential (=gate potential) is made higher. Thus, since a voltagebetween the back gate and source becomes higher to result in arelatively higher bias to the back gate, the threshold value of the NMOStransistor is increased. Similarly, when the CSP makes a leveltransition from the VDL to VDL′ (<VDL), the threshold voltage Vt of thePMOS transistor is increased. Thus, the threshold voltages Vt of theNMOS and PMOS transistors are increased through level transitions on theCSN and CSP. Therefore, the subthreshold leak current which determines adegree of SA current leakage can be decreased for reduction in leakcurrent from the VDL to VDBH. For attaining the above advantageouseffect, the sixth preferred embodiment is characterized in that there isprovided means for changing the levels of the common sources CSN and CSPamong the standby state, active state, and active-standby state.

[0103] In lieu of the P-side and N-side restore switches indicated inFIG. 3, Zp and Zn are used in the circuit scheme shown in FIG. 15. TheZp and Zn are means for supplying P-side and N-side restore potentialsand for changing these restore potentials according to control signals.As an example, the function of the Zn in operation is mentioned below.In the initial amplification period of sense amplifier operation,over-driving of the CSN is performed by the QDN1 using the VDBL. Afterthe over-driving is stopped, the Zn supplies a restore potential VDBH tothe CSN according to an SN control signal. Then, after a lapse of apredetermined period of time, the active-standby state is set up. Inthis state, the Zn drives the CSN to the VDBH′(>VDBH) according to anSN3 control signal.

[0104] Referring then to FIGS. 16 (a) to (d), there are shown examplesof arrangements of the Zn indicated in FIG. 15. In the Zn arrangementshown in FIG. 16 (a), a high-Vt NMOS QDN3 switch is added between theCSN and VDBH in parallel with the QDN. The QDN3 comprises alow-drive-power transistor having a gate length-to-width ratio W/L of1/500 or less of that of the QDN. When the QDN3 is activated, the QDN3supplies the VDBH′ (>VDBH) to the CSN. More specifically, the SN3provides a high impedance even when it is put into conduction, and avoltage drop takes place when a leak current of the sense amplifier isfed. Therefore, the CSN is set to the VDBH′ for reducing leakage by anegative feedback effect. The substrate potential of the QDN3 is set toa level equal to that of the QDN. Under condition that the senseamplifier is active, the QDN3 is activated when at least the QDN isinactive. The QDN3 may be activated simultaneously with the QDN in theinitial period of sense operation. For activating the QDN3, the SN3 isset from the VDBH to the VDL.

[0105] In the Zn arrangement shown in FIG. 16 (b), a low-Vt PMOS QDN3switch is added between the CSN and VDBH in parallel with the QDN. Whenactivated by a gate signal SN3, the QDN3 supplies the CSN with a powervoltage which is higher than the VDBH by the amount of Vt of the QDN3.The substrate potential of the QDN3 is set to a level equal to the VDLor a level equal to a potential at the PMOS of the SA. Under conditionthat the sense amplifier is active, the QDN3 is activated when at leastthe QDN is inactive. For activating the QDN3, the SN3 is set from theVDL to the VDBH.

[0106] In the Zn arrangement shown in FIG. 16 (c), a high-Vt NMOS QDN3switch using the VDBH′ as a power voltage is connected to the CSN.Therefore, this circuit scheme is based on the premise that a VDBH′(>VDBH) power supply circuit is formed. The VDBH′ power supply circuitcomprises such element circuits as a resistance divider circuit and avoltage limiter circuit. The substrate potential of the QDN2 is set to alevel equal to the substrate potential of the QDN. When the QDN3 isactivated by the SN3, the QDN3 supplies the VDBH′ to the CSN. Undercondition that the sense amplifier is active, the QDN3 is activated whenthe QDN is inactive. For activating the QDN3, the SN3 is set from theVDL to the VDBH.

[0107] In the Zn arrangement shown in FIG. 16 (d), a gate voltage of theQDN is controlled by the SN, i.e., the effect of the Zn is realizedusing the QDN. Through control of the gate signal SN3, an ON resistanceof the QDN is increased in the active-standby state for setting a CSNlevel to the VDBH′. In the circuit scheme shown in FIG. 16 (c), since noadditional transistor is provided, SN control operation becomes morecomplex than in the other examples of Zn arrangements. However, thecircuit scheme in FIG. 16 (c) is advantageous in that the peripheralcircuit layout of the sense amplifier can be simplified.

[0108] Referring to FIGS. 17 (a) to (d), there are shown examples ofarrangements of the Zp. Based on FIGS. 16 (a) to (d), these circuitschemes are modifications for P-side high level operation. The circuitschemes shown in FIGS. 17 (a) to (d) will be understood as in the abovedescription of those shown in FIGS. 16 (a) to (d).

[0109]FIG. 18 shows operating waveforms in an application where the Znand Zp arrangements shown in FIG. 16 (c) and FIG. 17 (c) are used in thesense amplifier circuit in FIG. 15. After input of a low-active command,the PCS makes a transition from the VDL to the VDBH, and the prechargeoperation is stopped. The operational sequence to be taken aftercompletion of the precharge until data is held in the sense amplifier isthe same as that described in Embodiment 2. Therefore, this operationalsequence is not described here. Under condition that data has beenestablished after completion of amplification through over-driving andrestore operations in the SA, a leak current is fed between the VDL andVDBH as mentioned in the foregoing. In a situation where a leak current“i” is fed per sense amplifier and “n” sample amplifiers are connectedto the common source line in the sub-memory array, the sum total ofcurrent leakage from the VDL to the VDBH is “ni”.

[0110] For reduction in current leakage, after a certain lapse of thelow-active signal, the sense amplifier amplifies a cell read-out signalto a sufficient level. Then, the SN and SP are deactivated, and the SN3and Sp3 are activated instead thereof. As a result, the CSN is set fromthe VDBH to the VDBH′, and the CSP is set from the VDL to the VDL′. Atthis step, the substrate potential of the NMOS transistor constitutingthe SA becomes relatively higher by (VDBH′-VDBH), and the substratepotential of the PMOS transistor also becomes relatively higher by(VDL-VDL′) . Through an effect of substrate biasing, the NMOS and PMOStransistors become to provide high Vt, thereby making it possible todecrease a sub-threshold leak current.

[0111] A minimum design value of amplitude (VDL′-VDBH) on a pair of datalines in the active-standby state is determined according to sensitivityof the sense amplifier. On the assumption that the data line amplitudeis 1.4 V, it is preferable to set the data line pair to approximately600 mV. Under this setting condition, even if a read command is applied,data is not destroyed to allow reduction in current leakage in theactive-standby state.

[0112] The following describes operations to be performed after input ofa precharge command for terminating the active-standby state. On inputof the precharge command, the SN3 and Sp3 are deactivated, and the SNand SP are activated, thereby rewriting the data line pair to have theVDBH or VDL. Then, the word line is deactivated to be set from the VPPto the VWL, and the SN and SP are deactivated. Finally, using the PCS,the data line pairs CSN and CSP are precharged to a precharge levelVDL/2

[0113] According to the present invention, it is also possible to attainan advantageous effect of reduction in current leakage on a prechargecircuit and a column switch comprising a low-Vt MOS transistor. In theactive-standby state, the precharge control signal PCS and Y selectsignals YS0 and YS1 are at any one of levels VDBH, VSS and VDBL. Wherethe NMOS substrate potential in the sense amplifier is used in common,an effect of substrate biasing is provided on an NMOS transistorconnected in series between the data lines included in the prechargecircuit PC, thereby increasing the Vt and applying a negativegate-source voltage. Thus, current leakage on the precharge circuit canbe reduced. This makes it possible to reduce current leakage between theVDL and VDBH. Similarly, by supplying the VDL/2 contained in theprecharge circuit PC, it is possible to reduce current leakage from theVDL/2 to the VDBH on the NMOS transistor connected with the lowlevel-side data line. Further, in a situation where the I/O line pairprecharge level is equal to or higher than the data line pair potential,current leakage on the NMOS transistor connected with the I/O line andlow-level-side data line can also be reduced.

[0114] It is to be understood that the present invention is not limitedin its application to a particular method of activating the CSN and CSPand a particular arrangement of MOS transistors to be activated foractivation of the sense amplifier. The present invention is alsoapplicable to an SA structure having a cross-coupled circuitconfiguration. For instance, for reduction in power consumption, thepresent invention is applicable to non-over-driving sense amplificationas well as over-driving amplification exemplified in Embodiments 1 to 5.

[0115]FIG. 19 shows an example in which the present invention isimplemented in a sense amplifier circuit of a non-over-driving type. Inthe circuit arrangement shown in FIG. 19, it is preferable to set thePMOS pair substrate potential of the sense amplifier to the VDL. TheQDP1 and QDP3 substrate potentials are also preferably set to the VDL.

INDUSTRIAL APPLICABILITY

[0116] Briefly described below are the advantageous effects to beattained in representative embodiments disclosed in the presentinvention. According to the present invention, in an over-driving senseamplifier circuit, a plurality of over-driving sense drivers aredistributively arranged in the sense amplifier circuit, thereby makingit possible to decrease a difference in common-source potential among aplurality of the sense amplifiers in sense operation. Further, for allthe sense amplifiers, an over-driving period can be controlled using agate signal, which provides an advantage that a difference in senseoperation with respect to far and near positions of the sense amplifierscan be reduced. Consequently, while ensuring high-speed sense operation,the present invention is capable of suppressing power consumption forimplementation of a low-power-consumption circuit.

1. A semiconductor device comprising: a plurality of sense amplifiersfor amplifying signals read out of a plurality of memory cells onto aplurality of data lines; a plurality of first MISFETs so arranged that afirst potential associated with a high level of an amplification signalfor said plurality of sense amplifiers is supplied from a first meshedpower line structure, each of said plurality of first MISFETs beingdisposed for a predetermined number of said plurality of senseamplifiers; and a plurality of second MISFETs so arranged that a secondpotential associated with a low level of an amplification signal forsaid plurality of sense amplifiers is supplied from a second meshedpower line structure, each of said plurality of second MISFETs beingdisposed for a predetermined number of said plurality of senseamplifiers; wherein said plurality of first and second MISFETs are ofthe same conduction type, and each gate of said first and second MISFETsis connected to a common drive control signal line; said drive controlline transmits a signal for controlling the conduction state of saidplurality of said first and second MISFETs; and an absolute value of thedifference between an effective level and an ineffective level is largerthan an absolute value of the difference between said first potentialand said second potential.
 2. A semiconductor device according to claim1 wherein said plurality of first and second MISFETs are arrangedalternately on a virtual line extending in one direction along saidplurality of sense amplifiers, and wherein said drive control signalline is formed along said virtual line and used to provide a gateelectrode for said plurality of first and second MISFETs.
 3. Asemiconductor device having a memory array including a plurality ofsub-memory arrays comprising: each of said plurality of sub-memoryarrays includes; a plurality of memory cells disposed at respectiveintersection points of a plurality of word lines extending in a firstdirection and a plurality of data lines extending in a second direction;a plurality of sense amplifiers disposed respectively in correspondencewith a plurality of said data lines, each of said plurality of senseamplifiers including a first MISFET pair of a first conduction typearranged in a cross-coupled form and a second MISFET pair of a secondconduction type arranged in a cross-coupled form; a first common sourceline extending to the first direction, said first common source linebeing coupled with sources of the first MISFET pair included in each ofsaid plurality of sense amplifiers; a second common source lineextending to the first direction, said second common sources line beingcoupled with sources of the second MISFET pair included in each of saidplurality of sense amplifiers; a first power line extending to the firstdirection, said first power line being supplied with a first potential;a second power line extending to the first direction, said second powerline being supplied with a second potential; a plurality of thirdMISFETs of the second conduction type disposed for a predeterminednumber of said plurality of sense amplifiers, a source-drain path ofsaid plurality of third MISFETs connected between said first commonsource line and said first power line; a plurality of fourth MISFETs ofthe second conduction type disposed for a predetermined number of saidplurality of sense amplifiers, a source-drain path of said plurality offourth MISFETs being coupled said second source line with said secondpower line; and a first drive control line extending to the firstdirection, said first drive control line being connected to gates ofsaid plurality of third and fourth MISFETs in common; and wherein thefirst MISFET pair included in each of said plurality of sense amplifiersis disposed along a first virtual line extending to the first direction;wherein the second MISFET pair included in each of said plurality ofsense amplifiers is disposed along a second virtual line extending tothe first direction; and wherein said plurality of third and fourthMISFETs are disposed along a third virtual line extending to the firstdirection between said first and second virtual lines.
 4. Asemiconductor device according to claim 3, wherein said plurality ofthird and fourth MISFETs are arranged alternately on said third virtualline.
 5. A semiconductor device according to claim 3, wherein each ofsaid plurality of sub-memory arrays has: a first area formed in a squareshape having first and second sides meeting each other at a cornercommon thereto, said first area having said plurality of word lines,said plurality of data lines, and said plurality of memory cells; asecond area formed along said first side of said first area, said secondarea having said plurality of sense amplifiers, said first and secondcommon source lines, said first and second power lines, and saidplurality of third and fourth MISFETs; a third area formed along saidsecond side of said first area, said third area having a plurality ofconnection parts for making connections between a plurality of worddrivers disposed respectively in correspondence with said plurality ofword lines or each of said plurality of word lines and said plurality ofword lines arranged in an upper layer; and a fourth area enclosed by acorner of said first area, a side of said second area, and a side ofsaid third area, said fourth area having a precharge circuit connectedto each end of said first and second common source lines.
 6. Asemiconductor device according to claim 3, wherein a signal read outfrom a memory cell onto said plurality of data lines is amplified to oneof said first potential and said second potential, and wherein, foractivation of a plurality of said sense amplifiers, there is provided aperiod of time that a voltage larger than a voltage between the firstpotential and the second potential is applied to said first drivecontrol line.
 7. A semiconductor device according to claim 3, whereineach of said plurality of sub-memory arrays includes: a third power linesupplied with a third potential; a fourth power line supplied with afourth potential; a fifth MISFET having a source-drain path connectedbetween one end of said first common source line and said third powerline; and a sixth MISFET having a source-drain path connected betweenone end of said fourth common source line and said fourth power line;wherein the third and fourth potentials are between the first potentialand the second potential, and a voltage between the first potential andthe second potential is larger than a voltage between the thirdpotential and the fourth potential, and wherein a signal read out ofsaid memory cell is amplified to one of the third potential and thefourth potential on one of said plurality of data lines.
 8. Asemiconductor device according to claim 7, wherein, after one of saidplurality of word lines is selected for amplifying a signal read fromsaid memory cell onto one of said plurality of data lines, saidplurality of third and fourth MISFETs are turned ON, and then after alapse of a predetermined period of time, said plurality of third andfourth MISFETs are turned OFF, and said fifth and sixth MISFETs areturned ON.
 9. A semiconductor device according to claim 7, wherein, forturning said plurality of third and fourth MISFETs ON, a voltage largerthan a voltage between the first potential and the second potential isapplied to said first drive control line.
 10. A semiconductor deviceaccording to claim 7, further comprising: a voltage boosting circuit forgenerating a boosted voltage to be applied to a selected one of saidplurality of word lines, wherein, for turning said plurality of thirdand fourth MISFETs OFF, said boosted voltage is applied to said firstdrive control line.
 11. A semiconductor device according to claim 3,wherein each of said plurality of sub-memory arrays has: a third powerline extending to the first direction, said third power line beingsupplied with a third potential; a fourth power line extending to thefirst direction; said fourth power line being supplied with a fourthpotential; a plurality of fifth MISFETs of the second conduction typedisposed for a predetermined number of said plurality of senseamplifiers, a source-drain path of said plurality of fifth MISFETs beingcoupled said first common source line with said third power line; and aplurality of sixth MISFETs of the second conduction type disposed for apredetermined number of said plurality of sense amplifiers, asource-drain path of said plurality of sixth MISFETs being coupled saidsecond common source line with said fourth power line; wherein the thirdand fourth potentials are between the first potential and the secondpotential, and a voltage between the first potential and the secondpotential is larger than a voltage between the third potential and thefourth potential, and wherein a signal read from a corresponding memorycell onto said plurality of data lines is amplified to one of the thirdpotential and the fourth potential.
 12. A semiconductor device accordingto claim 11, wherein, after one of said plurality of word lines isselected for amplifying a signal read from said memory cell onto one ofsaid plurality of data lines, said plurality of third and fourth MISFETsare turned ON, and then after a lapse of a predetermined period of time,said plurality of third and fourth MISFETs are turned OFF, and aplurality of fifth and sixth MISFETs are turned ON.
 13. A semiconductordevice according to claim 11, wherein, for turning said plurality ofthird and fourth MISFETs ON, a voltage larger than a voltage between thefirst potential and the second potential is applied to said first drivecontrol line.
 14. A semiconductor device according to claim 11, furthercomprising: a voltage boosting circuit for generating a boosted voltageto be applied to a selected one of said plurality of word lines,wherein, for turning said plurality of third and fourth MISFETs ON, saidboosted voltage is applied to said first drive control line.
 15. Asemiconductor device according to claim 3, wherein the first conductiontype is a P type, and the second conduction type is an N type.
 16. Asemiconductor device according to claim 3, wherein each of saidplurality of memory cells is a dynamic memory cell having one MISFET andone capacitor.
 17. A semiconductor device comprising: a plurality ofmemory cells disposed at respective intersection points of a pluralityof word lines and a plurality of data lines; a plurality of senseamplifiers disposed respectively in correspondence with said pluralityof data lines, each of said plurality of sense amplifiers including afirst MISFET pair of an N conduction type arranged in a cross-coupledform and a second MISFET pair of a P conduction type arranged in across-coupled form; a first common source line connected in common tosources of the first MISFET pairs included in said plurality of senseamplifiers; second common source line connected in common to sources ofthe second MISFET pairs included in said plurality of sense amplifiers;first drive means coupled said first common source line with a firstpotential; and second drive means coupled said second common source linewith a second potential; wherein said first and second drive meansprovide first and second operation modes, wherein said first drive meanssets up a connection between the first potential and said first commonsource line at a first impedance in the first operation mode and aconnection between the first potential and said first common source lineat a second impedance higher than the first impedance in the secondoperation mode, wherein said second drive means sets up a connectionbetween the second potential and said second common source line at athird impedance in the first operation mode and a connection between thesecond potential and said second common source line at a fourthimpedance higher than the third impedance in the second operation mode,and wherein, in a state that each of said plurality of sense amplifierslatches a signal from a corresponding one of said plurality of memorycells, a current fed to each of said plurality of sense amplifiers inthe second operation mode is smaller than a current fed thereto in thefirst operation mode.
 18. A semiconductor device according to claim 17,further comprising: means for supplying a first substrate bias, which isequal to or larger than the first potential, to a back gate of the firstMISFET pair in the first and second operation modes, and means forsupplying a second substrate bias, which is equal to or lower than thesecond potential, to a back gate of the first MISFET pair in the firstand second operation modes.
 19. A semiconductor device according toclaim 17, wherein threshold voltages of the first and second MISFETpairs in the second operation mode are larger than threshold voltagesthereof in the first operation mode.
 20. A semiconductor deviceaccording to claim 17, wherein said first drive means includes a firstswitch and a second switch disposed in parallel between said firstcommon source line and the first potential, wherein said first switch isselectively put into conduction in the first operation mode, and saidsecond switch is selectively turned on in the second operation mode, andwherein a conductance of said first switch is larger than a conductanceof said second switch.